Legislation requires that noxious emissions of internal combustion engines be treated. Selective catalytic reduction (SCR) uses ammonia as a reducing agent to turn nitrogen oxides (NOx) into nitrogen and water. Typically urea is injected periodically into the engine exhaust system upstream of an SCR chamber, where it mixes with hot exhaust gas to produce ammonia. This ammonia is absorbed onto the SCR catalyst in the chamber, and is effective at treating NOx. Over time the ammonia becomes depleted and a further injection of urea is required to recharge the catalyst. Depletion is estimated, usually in an engine management system, by monitoring engine activity over time. Systems for producing an injecting ammonia are also known.
An SCR catalyst is usually relatively remote from the engine; in a diesel engine it is for example downstream of the usual diesel oxidation catalyst (DOC) and diesel particle filter (DPF). As a consequence exhaust gas passing through the SCR chamber is often relatively cool, especially in city driving where vehicle speeds are low.
At low exhaust gas temperatures, thermal decomposition of urea to ammonia does not take place, and accordingly urea injection is inhibited by a control system. Such inhibition avoids formation of solid deposits in the chamber, and wastage of urea, but inevitably prolonged city driving may eventually result in a fully depleted catalyst. In the case of ammonia injection, deposits may also form at low exhaust gas temperatures.
It is against this background that the present invention has been conceived. Embodiments of the invention may provide an improved method or apparatus for treating exhaust emissions. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.